Laxity: Physiologic translation of the humeral head on glenoid

Instability: Pathologic translation of the humeral head on the glenoid causing pain or dysfunction

Posterior instability less common than anterior instability: 2–10% of shoulder dislocations

However, 50% of posterior shoulder dislocations seen in the Emergency Department are missed on initial evaluation.

Glenohumeral joint resembles a ball on tee:

1. 
   

   Articulating surface of humeral head is 3× larger than the surface of the glenoid.

Glenoid:

1. 
   

   Glenoid is pear-shaped, broader inferiorly than superiorly

   
   
2. 
   

   Provides 50% of the depth of the glenohumeral joint:

   
   
   
   
   1. 
      

      Labrum provides the other 50% of depth.

   
   
3. 
   

   Slightly concave:

   
   
   
   
   1. 
      

      Cartilage thicker at the periphery, bare spot centrally.

   
   
4. 
   

   Retroversion and inclination varies widely, approximately 0–5 degrees retroverted, 5 degrees inclined.

Humerus:

1. 
   

   Greater and lesser tuberosities are sites of rotator cuff insertion

   
   
2. 
   

   Retroverted 30 degrees from the transepicondylar axis, 130 degrees neck–shaft angle

Labrum:

1. 
   

   Provides 50% of the depth of glenohumeral joint

   
   
2. 
   

   Increases glenohumeral contact

   
   
3. 
   

   Provides conforming seal:

   
   
   
   
   1. 
      

      Negative intra-articular pressure.

      

Capsule:

1. 
   

   Posterior capsule only a few millimeters thick.

Ligaments:

1. 
   

   Inferior glenohumeral ligament:

   
   
   
   
   1. 
      

      Posterior band is primary restraint to posterior translation with the shoulder in abduction and internal rotation (pressing activity, follow-through phase of throwing).

   
   
2. 
   

   Superior glenohumeral ligament:

   
   
   
   
   1. 
      

      Primary restraint to inferior translation with the arm in adduction.

   
   
3. 
   

   Coracohumeral ligament:

   
   
   
   
   1. 
      

      Primary restraint to posterior translation with the arm in forward flexion, adduction, and internal rotation (fall on outstretched hands).

   
   
4. 
   

   Middle glenohumeral ligament:

   
   
   
   
   1. 
      

      Secondary restraint to posterior translation with the arm in adduction.

Musculature:

1. 
   

   Provide dynamic stabilization by compression into the glenoid concavity:

   
   
   
   
   1. 
      

      Rotator cuff:

      
      
      
      
      • 
        

        i. Subscapularis:

        
        
        
        
        • 
          

          Provides dynamic stabilization against posterior translation

          
          
        • 
          

          Primary restraint to posterior translation in external rotation.

        
        
      • 
        

        ii. Supraspinatus

        
        
      • 
        

        iii. Infraspinatus

        
        
      • 
        

        iv. Teres minor.

      
      
   2. 
      

      Other contributors: Teres major, latissimus dorsi, long head of biceps brachii, pectoralis major, and deltoid.

Posterior instability is generally acute or chronic:

1. 
   

   Acute etiologies:

   
   
   
   
   1. 
      

      Direct trauma:

      
      
      
      
      • 
        

        i. 50% of cases missed in Emergency Department

        
        
      • 
        

        ii. Posterior directed force on the glenohumeral joint.

      
      
   2. 
      

      Seizures or electrocution:

      
      
      
      
      • 
        

        i. Intense muscle contraction dislocates humeral head

        
        
      • 
        

        ii. Anterior dislocations more common with seizures, but proportion of posterior dislocations is higher.

   
   
2. 
   

   Chronic etiologies:

   
   
   
   
   1. 
      

      Microtrauma causing posterior capsule attenuation and stretching, leading to instability:

      
      
      
      
      • 
        

        i. Commonly presents as vague, deep pain with activity, not as frank instability

        
        
      • 
        

        ii. At risk activities include impact or load-bearing with the shoulder in a flexed and adducted position

        
        
      • 
        

        iii. Commonly seen in weightlifters (bench press), offensive linemen, some overhead athletes, and pitchers.

   
   
3. 
   

   Glenoid hypoplasia or extreme retroversion as a cause of instability is rare:

   
   
   
   
   1. 
      

      Congenital defects such as Erb’s palsy may be at risk

      
      
   2. 
      

      May predispose to recurrent instability or labral tears.

Posterior Bankart lesion:

1. 
   

   Detachment of the posteroinferior labrum from the glenoid at its inferior glenohumeral ligament attachment

   
   
2. 
   

   Pathognomonic for posteroinferior instability.

Posterior Bony Bankart lesion:

1. 
   

   Posteroinferior glenoid rim avulsion/shear fracture often in association with above findings.

Kim lesion:

1. 
   

   Avulsion of the deep posteroinferior labrum as above but with superficial labrum intact to glenoid.

Posterior humeral avulsion of glenohumeral ligament (HAGL)

1. 
   

   Avulsion of the posterior band of the inferior glenohumeral ligament from the humerus.

Posterior labral cyst:

1. 
   

   Cyst forms with valve-like effect of synovial fluid passing through labral defect

   
   
2. 
   

   Commonly seen in chronic posterior instability presentations.

Reverse Hill-Sachs lesion (▶ Fig. 13.2 ):

1. 
   

   Impaction fracture in the anterosuperior humeral head

   
   
2. 
   

   Head impaction results from contact with glenoid rim

   

   
   
3. 
   

   Pathognomonic for posterior dislocation

   
   
4. 
   

   May engage the glenoid and cause catching, recurrent dislocation or subluxation, or irreducibility.

Lesser tuberosity fracture:

1. 
   

   May present with acute/traumatic posterior dislocations.